A tensioning device of this type is known from German Utility Model DE-GM 7 412 059. In the German Utility Model, the base plate to which the idler wheel is attached is rather large, allowing the bolts for attachment of the base plate to an engine block to be placed outside the wheel surface. When space allowance is limited, the base plate must be reduced. When special requirements exist, space will only be available underneath the idler wheel. As a result, the servicing of the bolts during installation will be awkward and complicated.